The present invention relates to the production of cellular fused silica, and more particularly to the production of cellular fused silica having a bimodal closed cell structure. The invention also relates to such cellular fused silica and to the compound boron oxynitride, which is especially useful as the cellulating agent employed in the method of the invention.
Fused silica possesses a number of highly desirable properties, such as relative chemical inertness and resistance to attack by moisture, high electrical resistivity, and impermeability to liquids and gases. It is particularly known for its desirable refractory qualities, including a low thermal coefficient of expansion, high temperature resistance and high thermal shock resistance. Accordingly, fused silica is an exceedingly useful material in many applications including, for example, chemical apparatus, thermocouple protection devices, components of electronic systems, furnace parts and the like.
Dense, relatively nonporous fused silica blocks and bricks have long been known, being useful for the construction of refractory linings and the like, especially in open hearth steel furnaces. Open cell fused silica, i.e., cellular fused silica containing a multiplicity of cells which are inter-connected, is also well-known as a thermal insulating material. By virtue of its cellular structure, it is superior to the dense silica material in respect of its lighter weight, and also in having a lower thermal conductivity, which renders it more effective for thermal insulation. However, open cell fused silica is generally limited to use in dry environments, since its interconnected cells permit penetration of liquids. Moreover, open cell fused silica is generally characterized by low compressive strength and modulus of rupture as compared with the dense material, the strength decreasing with decreasing bulk density, thus precluding the use of open cell fused silica in many structural applications. For example, open cell fused silica having a bulk density of 0.5 g./cc. may have a compressive strength of only about 450 psi. and a modulus of rupture of only about 150 psi.
Closed cell silica, i.e., cellular silica wherein most or all of the cells are noncommunicating, has heretofore been produced, overcoming the disadvantageous permeability of the open cell type, but such materials have heretofore been characterized by poor mechanical strength, just as the open cell type. For example, U.S. Pat. Nos. 2,890,126 and 2,890,127 disclose an improved method of producing cellular silica with a closed cell structure, but the highest compressive strength reported therein is 125 psi. Further, due to the use of carbon and silicon carbide containing foaming agents, the silica foam thus made is frequently quite dark in color, often approaching black. This greatly degrades thermal insulating ability, due to increased thermal conductivity.